Despite the fact that the latest version of macOS (macOS Mojave) has been out for long, some Mac users still use some of the earlier versions of macOS. Among these are macOS High Sierra, Sierra, El Capitan, etc. In order to keep these updated, just follow the steps below:
Open the App Store on your Mac monitor.
In the App Store toolbar click Updates.
Use the Update buttons to download and install the updates listed there.
When there are no more updates in the App Store, your version of macOS and all of its apps are updated.
If you want to update your Mac automatically in the future, just select: Apple menu > System Preferences, click App Store, and select ”Download newly available updates in the background.”
In this way, your Mac will notify you every time when there are any updates to be installed.